If you are just focussing on your class twelfth board....then I would suggest that you drop a year. However, beforehand...you must thoroughly go through the syllabus and question papers of the past years.
This will help you to analyse how much time you need to give for each topic, you will be able to assess the difficulty level of the exam. You will also get to know at what stage would you like to join the coaching.
I would recommend that first you do self study and then join the coaching. If you are dropping a year to appear for the JEE exam, then your aim should be to finish the complete syllabus and be thorough with it by October 2018. After that you should start taking mock tests. By December you should be able to identify your weak and strong areas and then work on it in the last three months. The last one month before the exam should be just revision and you should not study anything new.